A known image forming system includes an image forming apparatus and an option device connected to the image forming apparatus. For example, an image forming apparatus of an image forming system is connected to option devices, e.g., a plurality of sheet supply devices configured to supply sheets to the image forming apparatus and a plurality of post-processing devices configured to perform processing for sheets having images formed thereon by the image forming apparatus, e.g., binding and fastening using staples. Each of the sheet supply devices and the post-processing devices includes a power supply unit configured to be connected to commercial power supply. Power is supplied to control circuits of the sheet supply devices and the post-processing devices from the power supply units. A drive circuit of a sheet supply device is configured to drive a motor and a solenoid with power supplied from a power supply unit.
The image forming apparatus is configured to transmit a control signal for turning power on and off, for example, to a sheet supply device, via a communication interface. A control circuit of the sheet supply device determines a content of a received control signal, and starts or stops power supply from the power supply unit of the sheet supply device to its drive circuit in response to the control content.